Geographic Variation in Leach's Storm-Petrel
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GEOGRAPHIC VARIATION IN LEACH'S STORM-PETREL DAVID G. AINLEY Point Reyes Bird Observatory, 4990 State Route 1, Stinson Beach, California 94970 USA ABSTRACT.-•A total of 678 specimens of Leach's Storm-Petrels (Oceanodroma leucorhoa) from known nestinglocalities was examined, and 514 were measured. Rump color, classifiedon a scale of 1-11 by comparison with a seriesof reference specimens,varied geographicallybut was found to be a poor character on which to base taxonomic definitions. Significant differences in five size charactersindicated that the presentlyaccepted, but rather confusing,taxonomy should be altered: (1) O. l. beali, O. l. willetti, and O. l. chapmani should be merged into O. l. leucorhoa;(2) O. l. socorroensisshould refer only to the summer breeding population on Guadalupe Island; and (3) the winter breeding Guadalupe population should be recognizedas a "new" subspecies,based on physiological,morphological, and vocal characters,with the proposedname O. l. cheimomnestes. The clinal and continuoussize variation in this speciesis related to oceanographicclimate, length of migration, mobility during the nesting season,and distancesbetween nesting islands. Why oceanitidsfrequenting nearshore waters during nestingare darker rumped than thoseoffshore remains an unanswered question, as does the more basic question of why rump color varies geographicallyin this species.Received 15 November 1979, accepted5 July 1980. DURING field studies of Leach's Storm-Petrels (Oceanodroma leucorhoa) nesting on Southeast Farallon Island, California in 1972 and 1973 (see Ainley et al. 1974, Ainley et al. 1976), several birds were captured that had dark or almost completely dark rumps. The subspecificidentity of the Farallon population, which at that time was O. l. beali, was thus problematic, because the presence of dark-rumped birds indicated inclusion in O. l. willetti (A.O.U. 1957). Not long after the capture of these birds, the A.O.U. (1973) synonymized O. l. willetti with O. l. beali. This action, following the recommendation of Austin (1952), "solved" the immediate ques- tion, but the problem seemedworthy of further pursuit, as Crossin (1974) found it impossibleto assignto any of five subspeciesmost specimenscollected away from breeding sites. He thought that most subspeciesof O. leucorhoa should be synon- ymized, a recommendation already made by Loomis in 1918. Van Rossem (1942), however, questionedthe wisdom of synonymizingall subspecies,and, exceptfor the recent decisionon O. l. willetti, the A.O.U. (1957, 1973) has followed van Rossem's subspecificdefinitions. The present study tests Loomis' (1918) and Crossin's(1974) hypothesis that Leach's Storm-Petrel in the eastern North Pacific is a dichromatic speciesin which the proportions of color phasesand mensural charactersvary geo- graphically in a cline, rather than there being the series of distinct populations currently recognized.I also compared vocalizationsamong birds from several lo- calities and tried to reveal those factors that might account for geographic variation in this species. METHODS Eight measurementsplus a classificationof rump color were made on 514 specimensor live individuals separatedinto 13 samples.The Farallon Island sample included 48 live birds mist-netted on the nights of 29-31 May 1978. All other samplesconsisted only of study skins. The rump color of an additional 164 skins from Guadalupe Island was classifiedin order to clarify the relationshipsof the summer and winter breeding populations there. Only specimenscollected on or immediately adjacent to a known breeding locality (i.e. the birds that flew aboard a boat anchored within a few hundred meters of shore) were used for measurement. Besides six standard measurements(Table 2), the shape (roundedness)of the wing tip was assessedby measuringthe distancefrom the tip of primary 10 to the tips of 9 and 7; the greater the 837 The Auk 97: 837-853. October 1980 838 DAVIDG. AINLEY [Auk, Vol. 97 Fig. 1. The seriesof referencespecimens (left to right, 1 to 11) showingcolor variation in O. leu- cotboa. differencein measurementthe more pointed the wing. Statistical significance between samples was tested usingDuncan's new multiple range test for a one-wayclassification of variance (Steel and Torrie 1960). Sampleswere compared at the 1% significancelevel. I found,as did Crossin(1974), that femaleLeach's Storm-Petrels were slightly larger than males but thatthe difference was not statistically significant without very large sample sizes (over 80 individuals), andit did notvary geographically. I thus combined the two sexesfor analysis;within all samplesthe sexeswere represented about equally. This combinationallowed me to useunsexed specimens, helped to increasesample sizes, and removedthe problemof usingincorrectly sexed specimens. Rumpcolor was classed by comparingspecimens with a seriesof 11reference skins. Each represented an approximatelyequal step in a continuumbeginning with all uppertail covertsentirely white, except for thefeather shafts, which may or maynot have been white (class 1), andending with all uppertail covertsentirely dark or concolorouswith the back and tail (class11). Specimens in the series (Fig. 1), all fromthe NationalMuseum of NaturalHistory, were as follows: 1 = USNM 201458,2 = 543662,3 = 132761,4 = 545030,5 = 544507,6 = 545031,7 = 543697,8 = 543694,9 = 543708,10 = 543726,11 = 543659.As for the intensityof a "bluishbloom" on the brownbody color, a characterthat wasgiven greatimportance in originaldescriptions of varioussubspecies and wasmost recently alluded to by Bourne(in Palmer 1962),I agreewholly with Crossin(1974) that it relatesto the extentof featherwear andthe ageof specimens(not the ageof thelive bird)rather than to geographicvariation. Recordingsof Leach'sStorm-Petrel vocalizations were made by R. L. DeLong,Pacific Ocean Biological SurveyProgram (POBSP), at theCoronado and San Benito islands in May 1967and at GuadalupeIsland in Juneand October1967, using a Uher Report-L4000 tape recorderand microphone.I recorded vocalizationsat the FarallonIslands in May 1975,using a SonyTC-800B recorder and an Altecmicro- phone.Sonagrams of Flight Calls and burrow Chatters were made at theMuseum of Vertebrate Zoology, Berkeley,using the wideaudio band on a Kay InstrumentCompany Sonagraph. Populationsat GuadalupeIsland.--Two populationsof O. leucorhoabreed at different seasonson GuadalupeIsland, but informationabout them, summarized by Crossin(1974), is fragmentary.I con- sideredspecimens collected on or nearthe islandfrom Octoberthrough April to be from the winter populationand thosecollected from May throughSeptember to be from the summerpopulation. This wasdeduced by comparingthe proportions of colormorphs from month to month(Table 1), a procedure suggestedby Crossin'sdiscovery that theseproportions seemed to differbetween the two populations. A clearseparation became obvious (Fig. 2):the colorsof summerbirds were spread between classes 1- 11but concentrated bimodally between 2-3 and7-10. Supportingthis separation were the facts that (1) theonly September specimens in the 10collections searched were recently fledged (collection dates were withinthe last few daysof that month);and (2) observationsof fledglings by otherpersons, summarized by Crossin,also occurred late in Septemberwith occasional dates as late as early November. It appears thatthe peak of fledging of summer chicks occurs in verylate September and early October. Extrapolating October1980] Leach'sStorm-Petrel Taxonomy 839 TABLE 1. The number of individuals of each color morph in samples of O. leucorhoa collected at Guadalupe Island. Colormorphs a Total speci- Months 1 2 3 4 5 6 7 8 9 10 11 mens October 3 6 12 10 6 1 1 39 November 1 3 5 7 1 17 December 1 4 4 10 8 3 31 January 2 2 4 4 4 1 17 February 1 1 3 2 3 2 1 13 March 1 (1) (1) 2 2 1 1 9 April 1 (1) 1 1 4 May 1 8 1 1 11 June 2 12 12 2 2 2 7 11 15 7 2 74 July 7 1 2 1 2 1 14 August 1 5 1 6 9 9 3 1 35 September (1) (1) (1) 3 Total 267 a Numbers in parenthesesare fledglings. from informationon breedingphenology at the Farallons(Ainley et al. 1974), egglaying of the summer breeding population of Guadalupe should occur from late May to the end of June. Crossin (1974: 174) surmisedthe egg-layingpeak to be late in the third week of June 1968, when the POBSP made a visit to Guadalupe. As for the winter population, information given by Crossin (1974: 174), plus the dates of three fledging specimensI inspected,suggested that most chicksfledge from early March to mid-April. The latest April specimenwas recently fledged and the other three April specimens(adults) were at a stage of molt or showed a degree of feather wear that, judging from information on molt in O. leucorhoa at the Farallons(Ainley et al. 1976),clearly placedthem in the winter breedingpopulation. Again using the Farallon data, egglaying in the summerpopulation of Guadalupeshould occur from mid-November through December. Crossin reported no eggspresent during a POBSP survey on 21 October 1967. The secondtype of information that supported my division of breeding populations is the fact that no adult specimensof O. leucorhoa have been collected(or at least were among the 267 specimensI ex- amined) at Guadalupe during the periods 25 April-28 May and 31 August through mid-October. This is consistentwith the suppositionthat May and October mark the initial month of the summerand winter breedingseasons, respectively. During thosemonths adults would make sporadicvisits at night to court and prepare nest sites and would not